This invention relates to a pulling apparatus for use in the production of semiconductor single crystals by a modification of the Czochralski method (hereinafter referred to as xe2x80x9cCZ methodxe2x80x9d for short), namely the so-called electromagnetic CZ method (hereinafter referred to as xe2x80x9cEMCZ methodxe2x80x9d) according to which the CZ method is carried out while rotating the melt by applying a magnetic field thereto and passing an electric current therethrough, and more particularly, to a single crystal pulling apparatus which can prevent the single crystal from falling due to heat generation in the seed portion upon current supply and can reduce the electric power consumption.
The CZ method is a typical method of producing single crystals for semiconductors, such as silicon single crystals. The CZ method comprises bringing a seed crystal into contact with the melt surface of a crystal material contained in a crucible and then rotating the crucible and pulling the seed crystal while rotating the same in a direction opposite to the direction of rotation of the crucible to thereby allow the single crystal to grow. In producing single crystals by the CZ method, dislocations generally occur due to thermal shock given at the time the seed crystal is brought into contact with the melt. To prevent these dislocations from extending to the body of the single crystal, the so-called xe2x80x9cDash""s neck processxe2x80x9d is employed.
FIG. 1 is a schematic representation of the shape of the neck portion (hereinafter referred to as xe2x80x9cDash""s neckxe2x80x9d) in the Dash""s neck process. As shown in FIG. 1, the Dash""s neck process in growing a single crystal comprises forming a Dash""s neck 4 having a diameter of about 3 mm and a length of about 30 mm on top of the body of growing single crystal 9 so that the above-mentioned dislocations may not extend to the body of the single crystal.
In recent years, single crystals themselves, which are pulled up, have become larger and larger and heavier and heavier to cope with the demand for increased efficiency in the manufacture of semiconductors. However, such increases in size of single crystals have induced problems such as mentioned below under (1) to (3).
Thus, (1) with the increase in size of single crystals, the pulling apparatus itself, inclusive of the crucible, becomes larger. Then, (2) it becomes difficult to rotate the present large-sized crucible at a constant number of revolutions without eccentricity relative to the pulling axis throughout the whole single crystal pulling process. Further, (3) during the single crystal pulling operation, the neck portion may be damaged due to the increased weight and the single crystal may fall down.
For solving the problems (1) and (2) mentioned above, Japanese Patent Application Laid-Open No. H11-171686 and Japanese Patent Application Laid-Open No. 2000-63195 (an improvement to the former) propose the EMCZ method which is characterized in that a magnetic field is applied to the melt and a current is passed therethrough to generate the Lorentz force and thereby rotate the melt within the crucible. The EMCZ method proposed in these publications makes it possible to grow single crystals in a smaller-sized pulling apparatus as compared with the conventional CZ method in which the rotation of the crucible itself is mechanically controlled. Further, it is advantageous that the impurity distribution within single crystals can be made uniform.
However, since the EMCZ method requires as a prerequisite that an electric current be passed through the seed crystal and the single crystal body, an increased amount of Joule heat is generated in the smaller diameter portion of the Dash""s neck, namely the portion indicated by a in FIG. 1, which shows a very high electric resistance. Due to this increased Joule heat generation, a local decrease in yield stress results. Thus, the single crystal pulling by the EMCZ method has the problem that, even when single crystals have a light weight, falling thereof tends to occur more readily as compared with the conventional CZ method.
Furthermore, as the electromagnetic force applied is increased, the rotation of the melt is disturbed and therefore the swing of the single crystal pulling axis increases. If the swing of the pulling axis increases, the single crystal may depart from the melt surface and it becomes difficult for the single crystal to grow steadily and stably. Furthermore, it becomes difficult to control the diameter of single crystal and, at the same time, further dislocations may occur, leading to decreases in the yield of grown single crystals and other problems.
In relation to the problem (3) mentioned above, the applicants have put into practical use a mechanism for preventing heavy single crystals from falling in the process of pulling by mechanically holding the engaging stepped portion having the form of an inverted cone and formed on top of the single crystal body (cf. Japanese Patent No. 2,990,658).
FIG. 2 is a schematic representation of the shape of a single crystal in applying the mechanical holding mechanism mentioned above. As shown in FIG. 2, for employing the above device in pulling single crystals, it is necessary to provide a large-diameter portion 5, an engaging stepped portion 6, a constricted portion 7 and a shoulder 8 between the Dash""s neck 4 and the single crystal body 9. The mechanism described in the above-cited Japanese Patent No. 2,990,658 mechanically holds the above engaging stepped portion 6 and thus can prevent falling with the Dash""s neck 4 as a starting point. This mechanism, however, is not intended for single crystal pulling by the EMCZ method, hence cannot solve the problems (1) and (2) mentioned above.
The present invention has been made in view of the problems mentioned above and it is an object of the invention to provide an apparatus by which even large-sized single crystals can be pulled safely, without allowing falling thereof, by inhibiting the local increase in electric resistance, namely the local increase in Joule heat, in single crystal pulling which is a problem intrinsic in the EMCZ method.
As a result of investigations concerning the problems of single crystal falling and the swing of the pulling axis in the EMCZ method mentioned above, the present inventors found that since, in growing single crystals by the EMCZ method, even a lightweight single crystal may fall during pulling, it is necessary to provide a mechanism for mechanically holding the single crystal and that shaft type pulling means for pulling single crystals is desirable for the prevention of single crystal swinging caused by the increased electromagnetic force.
It was further found that for reducing the local generation of Joule heat, it is effective to reduce the electric resistance of the Dash""s neck and of the engaging stepped portion by providing the so-called bypass in addition to the ordinary power supply route, that the electric resistance in the system fed with electric power can be reduced by providing the bypass mentioned above and, as a result, the power consumption can be reduced and that since a large current can be passed through the system, a sufficient electromagnetic force can be secured for rotating the melt even when the magnetic field, which requires a considerable power consumption, is weakened.
The present invention has been completed based on the above findings and is directed to the single crystal pulling apparatus summarized below under (1) to (3):
(1) A single crystal pulling apparatus for use in growing single crystals by the CZ method while applying a magnetic field and electric current to or through the semiconductor melt which comprises a main pulling means for pulling a single crystal, a holding mechanism for gripping a constricted portion formed on the single crystal through engaging members and a sub pulling means for moving the holding mechanism up and down and is characterized in that an electric current is passed through the main pulling means and through the sub pulling means (hereinafter, xe2x80x9cfirst apparatusxe2x80x9d).
(2) A single crystal pulling apparatus as defined above under (1) which is characterized in that after holding of the engaging stepped portion by the engaging members, the electric current through the main pulling means is shut off and an electric current is passed through the sub pulling means alone (hereinafter, xe2x80x9csecond apparatusxe2x80x9d).
(3) A single crystal pulling apparatus as defined above under (1) or (2) which is characterized in that the single crystal pulling means is of a shaft type (hereinafter, xe2x80x9cthird apparatusxe2x80x9d).